Self sacrifice sucks as a flaw when it's a positive trait forcefully twisted into a flaw. Like oh I'm too noble and selfless and good.
It's interesting when it's a symptom of a lack of self-worth that extends to many aspects of their life, which actually IS aggravating to watch.
Having your main character fatal flaw be "self-sacrificial" is kind of like when during a job interview they ask you for your biggest weakness and you say that you work too hard